No auto transtion occured from post-function

We are running JIRA 5.0.3 and script runner plugin v.2.07. I made a script post-function "fast-track transition on issue" in my workflow. No conditions in script - this fast-track must occure everytime the issue gets this transition. The needed transition has no screens, validators or conditions. However, sometimes this function does not work - in that case, I see this in the logs:

2014-06-05 17:01:08,817 Thread-1285 ERROR Username 1021x165720x2 13ui1rm 192.168.ip.address /secure/CommentAssignIssue.jspa [groovy.canned.utils.WorkflowUtils] Errors: {}Error Messages: [It seems that you have tried to perform a workflow operation (InWork) that is not valid for the current state of this issue (ISSUE-192). The likely cause is that somebody has changed the issue recently, please look at the issue history for details.]

No one is trying to start this transition manually, still this function doesn't work.

Sometimes this function works correctly, sometimes not - but I can't get, why.

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